In recent years, wireless local area network such as WiFi network has been widely deployed for both private home users and in public hot spots, so as to provide broadband high speed wireless access service to mobile terminals. In the wireless local area network, a single access point (AP) device supporting one or more mobile terminals is referred to as basic service set (BSS), wherein the AP device is identified by basic service set ID (BSSID). Two or more BSS form an extended service set (ESS). Usually one ESS is a single logic network segment (one ESS is within one IP subnet) and is identified by its service set ID.
The common WLAN architecture can be classified as autonomous network architecture and centralized network architecture. In the autonomous WLAN architecture, the AP device is independently managed as a separated network entity and has the function of providing accessibility to the mobile terminals. In the centralized WLAN architecture, it is provided with an access controller (AC) that configures, manages and controls the AP devices and cooperates with the AP devices to provide wireless accessibility to the mobile terminals. In the centralized WLAN architecture, the AP device is also referred to as wireless terminal point (WTP) device. The communication between the access controller AC and the WTP devices it controls utilizes the Control and Provisioning of Wireless Access Points (CAPWAP) protocol, in which two types of MAC models of the WTP device in the centralized WLAN architectures are defined: split MAC model and local MAC model. When the split MAC model is used, the WTP device is mainly used to receive and transmit wireless traffic without processing the received traffic, and the access controller AC performs all operations related to access. When the local MAC model is used, the WTP device executes all the wireless MAC functions.
In order to enable the mobile terminal to move between the different AP devices, Inter Access Point Protocol (IAPP) is proposed and is intended to provide function of moving between the AP devices to the mobile terminal, so as to satisfy mobile terminals' users' increasing need for mobility. However, the IAPP protocol is only applicable to intra-network roaming for the mobile terminal between all the AP devices within one ESS, wherein these AP devices are within the same network segment. According to the IAPP protocol, when the mobile terminal has been successfully associated with a target AP device, the target AP device will send a multicast packet containing MAC address of the mobile terminal to other AP devices in the ESS, so that the other AP devices can update their respective association tables.
Since the IAPP protocol can only enable the mobile terminal to perform the intra-network roaming between all the AP devices within one ESS, mobile IP (MIP) technology is introduced to ensure that the mobile terminal can perform inter-network roaming between different ESSes. The MIP technology enables the mobile terminal to use a fixed IP address to roam between the different IP segments. The MIP technology includes a series of processes such as mobile detection and network registration etc. When the mobile terminal moves from one ESS to another ESS, the original mobile agent server and the target mobile agent server need to exchange information. Generally, the mobile agent server is a general name for home agent (HA) server and foreign agent (FA) server. First, the HA server for the mobile terminal needs to be found, then the original FA server for the mobile terminal is identified by querying a binding table of the HA server, thereby establishing communication between the target FA server and the original FA server. However, the MIP handover would cause handover delay and packet loss, thereby reducing the service quality of the communication traffic (especially the real-time communication traffic).
The MIP technology and the IAPP protocol should be combined to provide a complete solution of roaming for the mobile terminal. Layer 3 MIP should support the IAPP protocol to implement the inter-network roaming. However, for the centralized WLAN architecture, the mobile agent discovery mechanism in the MIP technology cannot work. As mentioned above, the communication between the AC and its WTP devices is based on the Layer 3 tunnel. So Layer 3 function should be supported in a WTP device for both the split MAC model and the local MAC model. However, in case of the MIP technology, the foreign agent should be set in the AC or other higher logic position instead of being set in the WTP device. According to the standard MIP protocol, the time to live (TTL) of mobile agent announcement (MAA) of the MIP is usually set as 1 to avoid multicast storm, which means that the mobile terminal cannot obtain the MAA message sent by the FA server set in the AC due to the Layer 3 isolation of the WTP device. Vice versa, the FA server also cannot obtain the mobile agent solicitation (MAS) message sent by the mobile terminal.